Video Editing stereo tracks
In the Video Sequence Editor, add both audio clips.

On the left audio strip, set the Pan property to -1 under the Strip settings > Sound. On the right audio strip, set the pan property to 1.

Keyframe the Volume property (hover over it and press I to set a key) from 0 to 1 to make the audio fade from mute to 100% audible.



While this may work for some cases, I'd advise you to use a audio editing software (like the freeware audacity).